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Effect of pH and Molecular Length on the Structure and Dynamics of Short Poly(acrylic acid) in Dilute Solution: Detailed Molecular Dynamics Study

By Dimitris G. Mintis (6654587) and Vlasis G. Mavrantzas (1432291)

Abstract

Long MD simulations are carried out using a detailed all-atom force field to investigate the effect of pH or, equivalently, degree of ionization α<sup>–</sup> (= 0, 50, 100%) and degree of polymerization <i>N</i> (= 20, 23, 46, 70, and 110) on the structure and dynamics of poly­(acrylic acid) (PAA) at infinite dilution. To ensure the validity and add to the reliability of our research conclusions, a systematic validation of several molecular mechanics force fields is performed. It is observed that the generalized AMBER force field in combination with the RESP charge fitting method best describes both the structural and dynamical behavior of PAA in comparison to experimentally obtained data. It is found that ⟨<i>R</i><sub>g</sub><sup>2</sup>⟩<sup>0.5</sup>changes with <i>N</i> as ⟨<i>R</i><sub>g</sub><sup>2</sup>⟩<sup>0.5</sup> ∼ <i>N</i><sup>ν</sup>, with ν = 0.27 at α<sup>–</sup> = 0% degree of ionization (acidic conditions), ν = 0.94 at α<sup>–</sup> = 50% degree of ionization (neutral conditions), and ν = 0.87 at α<sup>–</sup> = 100% degree of ionization (basic conditions), which is in perfect agreement with theory. The global shape of the PAA chain in the solution is quantified in terms of the three eigenvalues of the average radius-of-gyration tensor, the relative shape anisotropy κ<sup>2</sup>, and the asphericity parameter <i>b</i>. It is revealed that at α<sup>–</sup> = 0%, the chain adopts a spherelike conformation, while at α<sup>–</sup> = 50 and 100%, its conformation is flattened and flexible. In addition, it is revealed that as the degree of ionization increases, the persistence length <i>L</i><sub>p</sub> increases, which suggests that PAA chains become stiffer with increasing pH. The global and local conformational changes of the PAA chain with the degree of ionization are found to be highly related to the solvation of the polymer. Finally, it is revealed that the diffusion coefficient <i>D</i> of the center of mass of PAA also exhibits a power law scaling with <i>N</i>, <i>D</i> ∼ <i>N</i><sup>ν</sup>, with ν = 0.25 at α<sup>–</sup> = 0% degree of ionization, ν = 0.46 at α<sup>–</sup> = 50% degree of ionization (neutral conditions), and ν = 0.44 at α<sup>–</sup> = 100% degree of ionization (basic conditions), in excellent agreement with recent experimental data and theoretical predictions

Topics: Biophysics, Biochemistry, Plant Biology, Biological Sciences not elsewhere classified, Chemical Sciences not elsewhere classified, Physical Sciences not elsewhere classified, PAA chain, mechanics force fields, shape anisotropy κ 2, all-atom force field, Molecular Dynamics Study Long MD simulations, ionization, asphericity parameter b, persistence length L p increases, AMBER force field, diffusion coefficient D, RESP
Year: 2019
DOI identifier: 10.1021/acs.jpcb.9b01696.s001
OAI identifier: oai:figshare.com:article/8063846
Provided by: FigShare
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